Protein-matrix solvation dynamics in the alpha subunit of C-phycocyanin

We have used femtosecond transient hole-burning (THB) spectroscopy to study single-chromophore dynamics at room temperature in the alpha subunit of C-phycocyanin, a cyanobacterial light-harvesting protein. The alpha subunit contains a single phycocyanobilin (open-chain tetrapyrrole) chromophore. A reasonable description of the THE spectra requires the presence of a broad excited-state absorption (ESA) line shape underlying the entire photobleaching/stimulated emission (PB/SE) region. The time evolution of the THB spectrum involves line broadening on the 100-200-fs time scale and a <100-fs time scale dynamic Stokes shift of the SE component. We suggest that the dynamic Stokes shift can be attributed to transient solvation by the protein matrix. The solvent correlation function, C(t), exhibits a sub-100-fs component that describes 90% of the total C(t) response, with the remaining 10% of response occurring on a much longer time scale than the longest delay employed in these experiments (10 ps), We assign the ultrafast decay component of C(t) to an inertial, librational response of the protein matrix. The slower component probably involves a diffusive, collective protein-matrix response.